A phase-locked loop (PLL) is an electronic circuit with an oscillator having an output that is constantly modified to match the frequency and phase of an input from another oscillator. In a phase-locked loop (PLL), a phase frequency detector compares the phase and frequency of a periodic signal (e.g., a clock) that is output by a variable frequency oscillator to the phase and frequency of a periodic input signal (i.e., a reference signal). Based on the comparison, the PLL adjusts the variable frequency oscillator to establish and maintain a constant phase relationship between the output signal and the input signal. Once the phase difference between the two signals becomes substantially constant, the PLL is said to be “in lock.”